Microwave heating apparatus



March 22, 1966 P. w. CRAPUCHETTES MICROWAVE HEATING APPARATUS Filed June 6, 1963 M/trawm/e o'ren -10- United States Patent 3,242,303 MICRGWAVE HEATING APPARATUS Paul Wythe Crapuchettes, Atherton, Calif assignor to Litton Precision Products, Inc., San Carlos, Calif. Filed June 6, 1963, Ser. No. 286,117 4 Claims. (Cl. 219-1055) This invention relates to microwave heating apparatus, and more particularly to means for preventing the escape of electromagnetic wave energy from a microwave heating cavity.

It is well known that high frequency electromagnetic wave energy can be used to heat and cook foodstuff or other lossy dielectric materials. There are devices commercially available which utilize such electromagnetic wave energy to cook food. Such devices are known as microwave ovens. These ovens consist of a conductive cavity to which electromagnetic wave energy is applied from a suitable source, usually a magnetron. The cavity of necessity includes an opening to provide access to the interior for placing food in and removing food from the oven. A suitably movable door, such as a hinged or sliding door, is provided to cover the opening during the time the microwave energy is being supplied to the interior of the oven. The door serves both to complete the cavity to allow proper cooking and to prevent the escape of electromagnetic wave energy to the exterior of the oven.

The electromagnetic wave energy must be confined to the interior of the oven for two reasons. The first, and probably more obvious, of these reasons is to prevent the escape of harmful radiation which may injure persons near the oven. The second reason is to prevent the escape of electromagnetic wave radiations which may interfere with the operation of other devices, such as communication devices, which utilize electromagnetic wave energy. With regard to this second reason, the Federal Communications Commission has assigned specific frequencies and wave lengths for use in microwave ovens and the escape of energy of this frequency would not interfere with any other communication apparatus. However, second and third harmonic frequencies are usually present within an oven and the escape of radiation of these frequencies would interfere with the operation of devices which have been assigned these frequencies by the Federal Communications Commission.

In the prior art, various means have been tried to prevent the escape of electromagnetic wave energy from the interior of a microwave oven. For example, in United States Patent 2,888,543, issued on May 26, 1959 to D. B. Haagensen, it is proposed that the interior surface of the cavity be coated with a solid dielectric material, such as a vitreous ceramic, to provide a relatively large capacitance between the door and the interior surface of the cavity. This capacitance would appear as a low impedance to microwave frequency energy and would short any energy which tried to escape from the cavity at the junction of the door and the cavity. However, such an arrangement has not proven to be entirely satisfactory in preventing the escape of radiation from the interior of the oven.

It is accordingly the object of the present invention to provide an improved microwave oven.

It is a further object of the present invention to provide improved means for preventing the escape of electro magnetic wave energy from the interior of a microwave oven through the junction of the door and the oven walls.

It is :a further specific object of the present invention to provide such an improved means which also prevents the escape of electromagnetic wave energy of any harmonic frequencies of the energy supplied to the microwave oven.

Briefly stated, and in accordance with an illustrative embodiment of the invention, a microwave oven is provided which includes an opening and 1a movably mounted door for closing the opening. When closed, the door is in spaced relation with an area of the outer wall of the oven. The dimensions of the opening and the door are chosen such that the area has a width of one quarter wave length of the second harmonic frequency of the electromagnetic wave energy supplied to the oven. A groove is provided in the area which has a depth of one quarter wave length of the fundamental frequency of the electromagnetic wave energy supplied to the oven. A resilient solid dielectric member is attached to the area to occupy the space between the outer wall and the door when the door is in its closed position. Two transmission lines are thereby formed, the first being the groove :and the second being the parallel surfaces of the outer wall and the door. The first transmission line forms a quarter wave length choke for the fundamental frequency and all odd harmonic frequencies thereof and the second forms a quarter wave length choke for all even harmonic frequencies of the electromagnetic wave energy supplied to the oven.

For a complete understanding of the invention, together with other objects and advantages thereof, reference may be had to the attached drawings, in which;

FIGURE 1 shows a side view of a microwave oven in which the door is in its closed position;

FIGURE 2 shows a sectional view of a portion of the oven of FIGURE 1; and

FIGURE 3 shows a front view of the oven of FIGURE 1 with the door in its opened position.

In the following description of the figures, like reference numerals are used to designate the same component in the different views wherever practical.

FIGURE 1 shows the side view of a microwave oven 10 to which electromagnetic wave energy from a suitable source 12, such as a magnetron and associated power supply, is furnished through a wave guide 14. The structure and operation of these devices are well known to those skilled in the art, and since they form no part of the present invention, no further description of them is given. The oven 10 includes a suitable opening (not shown in this view) through which access is available to the interior, and a door 16 pivoted on hinges 18 is provided to close this opening whenever the oven 10 is utilized. The door 16 includes a handle 20. FIGURE 1 also shows a solid dielectric member 22 positioned between the door 16 and the front of oven 10 when door 16 is in its closed position. Member 22 may be constructed from any suitable solid dielectric material which can withstand the cooking temperatures, such as tetrafluoroethylene, readily available under the trade name Teflon. The purposes :and advantages of dielectric member 22 will become apparent from the following description.

FIGURE 2 shows a sectional view of a portion of the oven of FIGURE 1. Shown therein is a portion of the door 16, dielectric member 22 and the front wall 24 of the oven 10. Door 16 is shown in its closed position to prevent the escape of electromagnetic wave energy from the interior 26 of oven 10 to the exterior 28 thereof.

As is shown in FIGURE 2, the boundary of the door 16 overlaps an area of front wall 24 when the door 16 is in its closed position. In the preferred form of the present invention, the width a of this area is made equal to a quarter wave length of the second harmonic frequency of the electromagnetic wave energy supplied to oven 10 from source 12.

In accordance with another feature of the preferred embodiment of the present invention, an L-shaped groove 30 is provided in front wall 24 which opens into the area of front wall 24 which is overlapped by the boundary of door 16 when door 16 is in its closed position. The

total depth [2 of groove 30 is made equal to a quarter wave length of the fundamental frequency of the electromagnetic wave energy provided to oven from source 12 and is thus also equal to three quarters wave length of the third harmonic frequency of this electromagnetic wave energy, five quarters wave length of the fifth harmonic frequency, and so forth for higher odd harmonic frequencies.

Dielectric member 22 is attached to the outer surface of front wall 24 to completely cover the area which is overlapped by door 16 when it is in its closed position. Thus, as will be readily appreciated by those skilled in the art, the adjacent conducting surfaces of door 16 and front wall 24 and the dielectric member 22 form a first parallel strip transmission line having a length a which is equal to a quarter wave length of the second harmonic frequency of the electromagnetic wave energy being supplied to the oven while the groove forms a second parallel strip transmission line having a length b equal to a quarter wave length of the fundamentalfrequency of the electromagnetic wave energy being supplied to the oven and having a length equal to three quarters wave length of the third harmonic frequency of this electromagnetic wave energy, and so forth for higher odd harmonic frequencies.

The operation of the invention is as follows: The second harmonic electromagnetic wave energy on the interior 26 of the cavity sees a parallel strip transmission line having a length equal to one quarter of its wave length and which is terminated in an open circuit. As is well known, an open circuit when viewed from a quarter wave length away appears to be a short circuit and thus the second harmonic energy sees a short circuit between door 16 and wall 24 and no energy of this frequency escapes between door 16 and front wall 24. This first parallel strip transmission line thus forms an open circuited choke for electromagnetic wave energy of this second harmonic frequency.

The same operation occurs for what may be termed the odd even harmonic frequencies, the sixth, tenth, and so forth harmonic frequencies. For these frequencies, this transmission line is an odd multiple of a quarter wave length long and thus the open circuited termination appears to be a short circuit on the interior side of the joint. However, what may be termed the even even harmonic frequencies, the fourth, eighth, and so forth, harmonic frequencies, the transmission line appears to he a multiple of a-half wave length long and thus, as is well known to those skilled in the art, the open circuited termination also appears to be an open circuit preventing the transmission of these even even harmonic frequencies. This transmission line thus forms a choke for all even harmonic frequencies of the electromagnetic wave energy supplied to oven 10.

, However, dielectric member 22 is transparent to electromagnetic wave energy and since the resulting parallel strip transmission line is not a quarter wave length of the primary or third harmonic frequency of the electromagnetic wave energy, energy of these frequencies can escape from the interior 26 and enter the transmission line. However, when this energy reaches the point at which groove 30 emerges from front wall 24, it sees the second parallel strip transmission line which is either a quarter wave length or three quarters wave length long and which is terminated in a short circuit. As is well known to those skilled in the art, a transmission line having a length of a quarter wave length, or any odd multiple thereof, which is terminated in a short circuit appears to be an open circuit from the input end. Thus, any energy of the primary or third harmonic frequency which reaches this point sees in series therewith what appears to be an open circuit and the energy is blocked and prevented from being further transmitted to the exterior 28 of oven 10. Groove 30 thus forms a second parallel strip transmission line which is a short circuited choke for electromagnetic wave energy of the fundamental or any odd harmonic frequency thereof.

FIGURE 3 shows a front view of oven 10 with the door 16 in its open position and shows the opening 32 through which access is had to the interior of oven 10. FIGURE 3 also shows dielectric member 22 completely surrounding opening 32. Not shown in the figure is groove 30 which is covered by dielectric member 22. Groove 30 also completely surrounds opening 32. The dimensions of opening 32 and door 16 are chosen such that the boundary of door 16 overlaps opening 32 over an area having the width a completely around the periphery of opening 32, and this area is completely covered bydielectric member 22, which is attached to front wall 24 of oven 10.

It is readily apparent to those skilled in the art that the dimensions a and b could be interchanged and that the resultant structure would still retain microwave energy on the interior 26 of oven 10. However, in practice it has been found that a short circuited choke such as groove 39 is a more efficient means of stopping electromagnetic wave energy than is the open circuited choke consisting of the parallel surfaces of door 16 and front wall 24 and dielectric member 22, and since the intensity of the primary frequency and third harmonic frequency is much greater than the second harmonic frequency, the preferred form of the present invention utilizes the more efficient short circuited choke to trap these components and the less efficient open circuited choke to trap the second harmonic frequency component of the electromagnetic wave energy.

In practice, it has also been found that the short circuited choke, groove 30, loses some of its effectiveness against higher odd harmonic frequencies because of the capacitance appearing across the opening in the groove 30. However, this same capacitance appears as a low impedance to these same higher odd harmonics and prevents their escape to the exterior of the oven, and thus the object of the invention is still satisfied.

In addition to retaining electromagnetic wave energy within the interior 26 of oven 10, there are several other advantages realized in the practice of the present invention. For example, dielectric member 22 maintains a uniform separation between door 16 and front wall 24 of oven 10. Thus, in addition to forming the above described transmission line, dielectric member 22 also prevents arcing between door 16 and front wall 24, such as has been experienced in oven in which the door makes a metal-to-met-al contact with the remainder of the cavity. Also, dielectric member 22 completely covers groove30 and thus prevents any food particles from becoming trapped in groove 30. Dielectric member 22 thus renders the oven easier to clean and gives a more attractive appearance to the oven. Also, dielectric member 22 acts as a muffler to cushion any noise from door 16 being closed too hard against oven 10. 7

It is to be understood that the above described arrangements are illustrative of the application of the principles of the present invention. Numerous other arrangements within the scope of the invention may be devised by those skilled in the art. Thus, by way of example, but not of limitation, groove 30 could instead be provided in door 16 rather than in front wall 24, in which case dielectric member 22 would be attached to door 16 to cover groove 30. The electrical properties of the resultant structure would be identical with the above disclosed arrangement when door 16 is in its closed position. Also, the shape of groove 30 need not be L-shaped, but might be any shape which provides the necessary length. Accordingly, from the foregoing, it is evident that various changes may be made in the present invention without departing from the spirit of the invention as defined in the appended claims. a

What is claimed is:

1. Microwave heating apparatus comprising, a heating cavity, means for supplying electromagnetic wave energy of a predetermined wave length and frequency to said cavity, said cavity having an opening therein for access to the interior thereof, a conductive surface extending away from the exterior side of said opening, a movably mounted door for closing said opening whenever said heating apparatus is to be used, the boundary of said door being positioned in spaced relation with an area of said conductive surface when said door is in its closed position, the width of said area being a first predetermined dimension with respect to said predetermined wave length, a groove formed in one of said area and said boundary, the depth of said groove being a second predetermined dimension with respect to said predetermined wave length, a solid dielectric member attached to said one of said area and said boundary, thereby covering the opening of said groove and occupying the space between said area and said boundary when said door is in its closed position, said area, boundary and dielectric member forming a first parallel strip transmission having a length equal to said first predetermined dimension, said groove form-ing a second parallel strip transmission line having "a length equal to said second predetermined dimen- $1011.

2. Microwave heating apparatus comprising, a heating cavity, means for supplying electromagnetic wave energy of a predetermined wave length and frequency to said cavity, said cavity having an opening therein for access to the interior thereof, a conductive surface extending away from the exterior side of said opening, a movably mounted door for closing said opening whenever said heating apparatus is to be used, the boundary of said door being positioned in spaced relation with an area of said conductive surface when said door is in its closed position, the Width of said area being one quarter wave length of the second harmonic of said predetermined frequency, a groove formed in one of said area and said boundary, the depth of said groove being equal to one quarter of said predetermined wave length, and a soliddielectric member attached to said one of said area and said boundary, thereby covering the opening of said groove and occupying the space between said area and said boundary when said door is in its closed position, said area. boundary, and dielectric member forming a first parallel strip transmission line having a length equal to one quarter wave length of the second harmonic of said predetermined frequency, thereby forming an open circuited choke for said second harmonic, said groove forming a second parallel strip transmission line having a length equal to one quarter of said predetermined wave length, thereby forming a short circuited choke for said predetermined frequency and the odd harmonic frequencies thereof.

3. Microwave heating apparatus comprising, a heating cavity, means for supplying electromagnetic wave energy of a predetermined wave length and frequency to said cavity, said cavity having an opening therein for access to the interior thereof, a conductive surf-ace extending away from the exterior side of said opening, a movably mounted door for closing said opening whenever said heating apparatus is to be used, the boundary of said door being positioned in spaced relation with an area of said conductive surface when said door is in its closed position, the width of said area being a first predetermined dimension with respect to said predetermined wave length, a groove formed in said area, the depth of said groove being a second predetermined dimension with respect to said predetermined wave length, and a solid dielectric member attached to said area, thereby covering the opening of said groove and occupying the space between said area and boundary when said door is in its closed position, said area, boundary, and dielectric member forming a first parallel strip transmission line having a length equal to said first predetermined dimension, said groove forming a second parallel strip transmission line having a length equal to said second predetermined dimension.

4. Microwave heating apparatus comprising, a heating cavity, means for supplying electromagnetic wave energy of a predetermined wave length and frequency to said cavity, said cavity having an opening therein for access to the interior thereof, a conductive surface extending away from the exterior side of said opening, a movably mounted door for closing said opening whenever said heating appanatus is to be used, the boundary of said door being positioned in spaced relation with an area of said conductive surface when said door is in its closed position, the width of said area being equal to a quarter wave length of the second harmonic of said predetermined frequency, a groove formed in said area and completely surrounding said opening, the depth of said groove being equal to one quarter of said predetermined wave length, and a solidv dielectric member attached to said area, thereby covering the opening of said groove and occupying the space between said area and boundary when said door is in its closed position, said area, boundary and dielectric member forming a first parallel strip transmission line having a length equal to a quarter wave length of the second harmonic of said predetermined frequency, thereby forming an open circuited quarter wave length choke for said second harmonic, said groove forming a second parallel strip transmission line having a length equal to one quarter of said predetermined wave length, thereby forming a short circuited quarter wave length choke for said predetermined frequency and the odd harmonic frequencies thereof.

References Cited by the Examiner UNITED STATES PATENTS 2,500,676 3/1950 Hall et al 219-1055 2,632,838 3/1953 Schroeder 219--10.55

2,958,754 11/ 1960 Hahn 21910.55

FOREIGN PATENTS 1,262,776 4/1961 France.

RICHARD M. WOOD, Primary Examiner. 

1. MICROWAVE HEATING APPARATUS COMPRISING, A HEATING CAVITY, MEANS FOR SUPPLYING ELECTROMAGNETIC WAVE ENERGY OF A PREDETERMINED WAVE LENGTH AND FREQUENCY TO SAID CAVITY, SAID CAVITY HAVING AN OPENING THEREIN FOR ACCESS TO THE INTERIOR THEREOF, A CONDUCTIVE SURFACE EXTENDING AWAY FROM THE EXTERIOR SIDE OF SAID OPENING, A MOVABLY MOUNTED DOOR FOR CLOSING SAID OPENING WHENEVER SAID HEATING APPARATUS IS TO BE USED, THE BOUNDARY OF SAID DOOR BEING POSITIONED IN SPACED RELATION WITH AN AREA OF SAID CONDUCTIVE SURFACE WHEN SAID DOOR IS IN ITS CLOSED POSITION, THE WIDTH OF SAID AREA BEING A FIRST PREDETERMINED DIMENSION WITH RESPECT TO SAID PREDETERMINED WAVE LENGTH, A GROOVE FORMED IN ONE OF SAID AREA AND SAID BOUNDARY, THE DEPTH OF SAID GROOVE BEING A SECOND PREDETERMINED DIMENSION WITH RESPECT TO SAID PREDETERMINED WAVE LENGTH, A SOLID DIELECTRIC MEMBER ATTACHED TO SAID ONE OF SAID AREA AND SAID BOUNDARY, THEREBY COVERING THE OPENING OF SAID GROOVE AND OCCUPYING THE SPACE BETWEEN SAID AREA AND SAID BOUNDARY WHEN SAID DOOR IS IN ITS CLOSED POSITION, SAID AREA, BOUNDARY AND DIELECTRIC MEMBER FORMING A FIRST PARALLEL STRIP TRANSMISSION HAVING A LENGTH EQUAL TO SAID FIRST PREDETERMINED DIMENSION, SAID GROOVE FORMING A SECOND PARALLEL STRIP TRANSMISSION LINE HAVING A LENGTH EQUAL TO SAID SECOND PREDETERMINED DIMENSION 